Multipass optical retroreflector and method of using

ABSTRACT

This invention pertains to a multipass retroreflector and to a method for its operation. In a preferred embodiment, the retroreflector comprises three reflecting surfaces arranged at an angle to each other in the form of a prism that is triangular in cross-section and 1 to 2 ports through which an optical beam enters and/or exits. The method includes the step of admitting an optical beam into the retroreflector where the optical beam is reflected from the reflecting surfaces and exits parallel to the path of the incoming optical beam. The method also includes the steps of spacing and adjusting angular disposition and dimensions of the reflecting surfaces in order to change path length of the optical beam passing through the retroreflector. This spacing can be continuously scanned causing a large change in optical path length due to a small change in the position of the moving surface.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention:

[0002] This invention pertains to the field of collinear andnon-collinear retroreflectors and to methods for their operation.

[0003] 2. Description of Related Art:

[0004] Multipass optical cells have been used for many years in thefield of gas and atmospheric absorption spectroscopy so that a smallvolume can be interrogated by making multiple passes through the volume.This achieves a long optical path length in a very compact structure, orsimilarly, amplifies via multiple passes a weak optical response of asingle pass.

[0005] Retroreflection of an optical signal is very important in opticalsystems since it not only reflects the light, but also causes the returnoptical path to be parallel to the incident path. This has manyapplications such as optical delay line scanning, optical alignment, andbright passive reflections such as road signs, to name a few.

[0006] Combining the trait of a multipass optical cell with aretroreflector creates a flexible tool for use in compact laseramplifier gain stages, communication, spectroscopy, and remote sensing.

[0007] Donald R. Herriott and Harry J. Schulte in an article entitled“Folded Optical Delay Lines” in the journal Applied Optics, for August1965, vol. 4, No. 8, starting on p. 883, disclose that a long opticalpath has been folded between two 7.5 cm diameter spherical or asphericalmirrors to provide an output optical beam which can be well separatedfrom previous reflections with 1000 or more passes between the mirrors.The 3000-m path provides 10 μsec of delay. This system can be used as adispersionless optical delay line for use in filtering or storage ofinformation modulated onto the light beam. The pattern of beams betweenthe two mirrors is obtained in one of two ways. A small perturbingmirror may be inserted to give a series of offset ellipses, or one orboth of the mirrors can be made astigmatic to give a Lissajous patternof spots on each mirror. The output beam can be separated from others bydiscriminating in both angle and position. The diffraction losses of thesystem are much lower than those for an open beam because of theperiodic focusing of the spherical mirrors. The extreme dependence ofthe loss of the delay line upon the absorption and scattering loss ofthe mirrors makes the system a suitable method for measuring mirrorloss.

[0008] U.S. Pat. No. 5,973,864 to Lehman et al discloses a stableresonator for a ring-down cavity spectroscopy cell having an optic axis.The resonator includes two Brewster's angle retroreflector prisms, eachhaving a plurality of total internal reflection surfaces. The prisms aredisposed in alignment along the optic axis of the resonator. One or bothof the prisms can be rotated so that light rays enter a surface of theprism nearly at Brewster's angle to the normal of the prism surface.This feature maintains alignment between the prisms and allows theresonator to be tuned. One of the internal reflection surfaces of atleast one of the prisms may be a curved surface. Alternatively, anastigmatic lens may be centered in one arm of the resonator and tiltedat Brewster's angle with respect to the optic axis of the resonator.

OBJECTS AND BRIEF SUMMARY OF THE INVENTION

[0009] An object of this invention is a collinear or non-collinearoptical retroreflector and method of operating or using same.

[0010] Another object of this invention is a simple and compact designfor a multipass optical cell.

[0011] Another object of this invention is a device that can be used asa stand-alone multipass optical retroreflector or as a scanning opticaldelay line.

[0012] Another object of this invention is an optical delay line devicethat can achieve large delays with small structural displacementstherein.

[0013] Another object of this invention is an optical retroreflectorthat is scalable.

[0014] These and other objects of this invention can be achieved by adevice that includes three or more cooperating reflective surfaces ormirrors and at least one non-reflective port that allows light to enterand/or exit after reflections from the reflective surfaces.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015]FIG. 1 is a cross-sectional view of an optical device or acollinear retroreflector.

[0016]FIG. 2 is a cross-sectional view of an optical device or anon-collinear retroreflector.

[0017]FIG. 3 is an isometric view of the retroreflector of FIG. 1.

[0018]FIG. 4 is a cross-sectional view of another collinearrotoreflector having a path length of an optical beam that is longerthan that of the retroreflector of FIG. 1.

[0019]FIG. 5 is a cross-sectional view of a collinear retroreflectorwherein optical delay line scanning causes small movement of a singlereflecting surface to be magnified creating a large change in opticaldelay.

DETAILED DESCRIPTION OF THE INVENTION

[0020] This invention combines the concepts of optical beamretroreflection and optical multipass cells. The invention discloses aretroreflecting optical geometry that reflects the incident optical beamfrom the reflecting surfaces back upon the identical incident path oranother path that is parallel to the incident path. Within theboundaries of this cell or device, the optical beam undergoes multiplereflections. The number of reflections within the cell is a function ofthe boundary angles, dimensions of the reflecting surfaces and theposition of the incident optical beam.

[0021] This invention describes a novel design to achieve a multipassoptical cavity that is naturally retroreflecting upon the exact path ofthe incident light or another path that is parallel thereto. This isachieved by placing, typically, three reflective surfaces in aprism-like geometry, while allowing a non-reflective break(s) or port(s)in one surface to allow the light to enter and exit. In one embodiment,the retroreflection is collinear, thereby causing the optical beam toenter and exit the retrorelector from the same port whereas in anotherembodiment, the retroreflection is non-collinear where the light oroptical beam enters and exits the retroreflector through differentports.

[0022] The concept of this invention is shown in FIG. 1 where acollinear retroreflector in the form of a a structure or prism 10 iscomposed of internally reflecting surfaces 12, 14, 16, which are nottransparent to a light beam. The reflecting surfaces are disposed at anangle to each other and, typically, abut and are fixed to each other atcorners 18, 20, 22 to form the triangular structure in cross-sectionexcept that at corner 18, there is a port or a gap 24 which forms aninlet through which a light beam from a light source enters the interiorof the structure. As will be later explained, more than one port in thestructure can be provided, typically at an edge of one of the reflectingsurfaces. Furthermore, the port can be a continuation of surface 12where a portion of the surface is transparent to light. Continuation ofsurface 12 can extend to where it abuts and is fixed to surface 14.

[0023] The reflective surfaces can be coated with optical modulators formodulation of the incident optical beam or thin-film materials foranalysis and/or evaluation.

[0024] It should be understood that surfaces 12, 14, 16, here and inother embodiments, need not be affixed to each other in this or otherembodiments of this invention, but can be loosely arranged and held inplace by outside means so that the desired reflections are obtainedthrough adjustments of the reflecting surfaces relative to each other orotherwise. For some spectroscopic applications, the surfaces must form afully enclosed, hermetic prism so that a fluid can be held therein andanalyzed by passing an optical beam therethrough.

[0025] A collinear retroreflector has one port where the inlet opticalbeam enters and the exit optical beam exits, after reflecting off thereflecting surfaces, along the same path as the inlet optical beam. Theport can be positioned anywhere, however, but it is typically positionedalong an edge of the surface 12.

[0026] A collinear retroreflector shown in FIG. 1 and having angle atcorner 18 of 45°, angle at corner 20 of 87°, and angle at corner 22 of48°, contains 23 reflections. If reflectivity of the reflective surfaces12, 14, 16 is 99%, optical losses due to reflectivity of 99%, is 21% .

[0027] The vertical reflecting surface or mirror 12 has port 24 at thebottom, but this port can be at the top or an aperture or opening can becreated in the center or anywhere else in the reflecting surface.Operation of or method of using the collinear retroreflector 10 of FIG.1 involves passing an optical beam along path 26 into retroreflector 10whereby the optical beam is reflected numerous times from the reflectingsurfaces 12, 14, 16 and then exits the retroreflector 10 along path 26.Source of the optical beam can be anything available, including a lightbulb or a laser. The method also allows for the continuous changing ofoptical path length, and thus of optical time delay, by moving orscanning one of the surfaces. The method also includes multiplereflections and the movement of one surface to cause a large change inoptical path length through a small change in the position of the movingsurface, thereby creating large changes in optical time delay.

[0028]FIG. 2 shows a non-collinear retroreflector 50 composed ofinternally reflecting surfaces 52, 54, 56 which are not transparent toan optical beam. The reflecting surfaces are disposed at an angle toeach other and, typically, abut and are affixed to each other at corners58, 60, 62 and along the edges of the reflecting surfaces except for thepresence of two ports 64, 66 through which an optical beam enters andexits from interior of the retoreflector. The reflecting surfaces form astructure that is triangular in cross-section. A non-collinearretoreflector is characterized by the fact that the optical beam entersalong one path but leaves the retoreflector by another parallel path.Ports 64, 66 can be a continuation of surface 52 but the surface must betransparent to the optical beam at ports 64,66.

[0029] Operation of the non-collinear retroreflector 50 of FIG. 2involves passing an optical beam (light) along path 68 through port 64into retroreflector 50 wherein the optical beam is reflected from theinteriorly reflecting surfaces 52, 54, 56 and then exits through port 66along path 70, which is parallel to path 68.

[0030]FIG. 3 is an isometric view of the retroreflector of FIG. 1showing the retroreflector in the form of a prism that is triangular incross-section. The retroreflector 10 comprises internally reflectingsurfaces 12, 14, 16 which form a prism with acute angle at corner 20 andacute angles at corners 18, 22. Port 24 is adjacent to corner 18 but canbe anywhere in the reflecting surface 12. The port can be in the form ofa slit which extends the length of surface 12 but it is typically in theform an opening of sufficient size to allow passage of an optical beam.Surfaces 12, 14, 16 are typically rectangular but can be of any othershape, planar or curved, depending on design of the retroreflector. Itshould be understood that rectangular figures include squares. Surface12 in FIG. 3 is shown as a line, however, it should be understood thatit is rectangular with its edge extending the width between referencenumbers 18 and 22 and in depth between reference numbers 22 and 22′ or18 and 18′ , except for port 24 which can be a slit extending the depthof surface 12 or a corner opening. Reference numerals 18′ , 20′ , and22′ denote the depth extent of the reflective surfaces starting atcorners 18, 20 , 22, respectively. Although three reflecting surfacesare shown for retroreflector 10, it should be understood that any numberin excess of three reflecting surfaces can be used to construct aretroreflector to have an optical beam reflect from the reflectingsurfaces for spectroscopic purposes or for increasing path length of anoptical beam in order to obtain the desired time delay forcommunications or any other application.

[0031] To increase the number of passes of the optical beam, the anglesare adjusted to increase the path length. FIG. 4 illustrates a collinearretroreflector 100 consisting of reflecting surfaces or mirrors 102,104, 106. Surface 102 is vertically disposed, lower surface 104 isdisosed upwardly at an acute angle to the vertical, and the uppersurface 106 is disposed downwardly at an acute angle from the vertical.Retroreflector 100 shown in FIG. 4 includes port 108 which can be a slitextending the extent of surface 102 but is typically a corner opening.Vertical surface 102 meets lower surface 104 at corner 110, in absenceof port 108, and upper surface 106 meets the vertical reflecting surface102 at corner 114. Upper surface 106 and lower surface 104 meet atcorner 112. As shown in FIG. 4, the reflecting surfaces formretroreflector 100 with an interior volume wherein an optical beam isreflected many times before exiting. Number of reflections for theretroreflector 100 is 59 if the corner angle from the vertical at corner110 is 45°; angle at corner 112 is 89°; angle at corner 114 is 46°.

[0032] For comparison, a particular embodiment of FIG. 1 has angles of45°, 48°, and 87° at corners 18, 22 and 20, respectively, and aparticular embodiment of FIG. 4 is similar since the angle at corner 110is 45°, similar to that of corner 18 in FIG. 1, but the angle at corner112 has increased to 89° from its corresponding 87° angle at corner 20in FIG. 1. This changes corner 116 in FIG. 4 to 46° from itscorresponding 48° angle at corner 22 in FIG. 1. Implementing thesechanges increases the optical path length in the FIG. 4 embodiment by145% over that in FIG. 1.

[0033]FIG. 5 illustrates a collinear retroreflector 150 which iscomposed of reflecting surfaces 152, 154, 156 and port 158, similar tothe retroreflector 100 of FIG. 4. Corner 160 is at bottom of theretroreflector 150 shown in FIG. 5, corner 162 is up and to the right ofcorner 160, and corner 164 is above corner 160. Retroreflector 150 ofFIG. 5 is the same as the retroreflector 100 of FIG. 4 with oneexception: surface 152 is scanned or displaced from the rest of theretroreflector by a distance X in order to create an optical time delaycaused by a small movement of the reflective surface which is magnifiedyielding a large change in optical delay. Displacement X is offset auniform distance from the retroreflector although it is typicallyscanned or continuously moved over the distance X. Although reflectingsurface 152 is spaced from the rest of the retrorefletor , the surfacesform a space, although the space may be hermetically enclosed using afixed transparent surface to the right of surface 152.

[0034] The device disclosed herein, i.e., a multiple passretroreflector, is useful for optical delay scanning since movement of asingle reflective surface or mirror is magnified by the multiplereflections off its surface creating a large change in optical delay fora small change in mirror position. For instance, if the vertical surface102 is 100 units in length, moving it 5 units to the left where X is 5units, produces a 199 unit change in path length. Thus, in an opticalmicro-electro mechanical system, or MEMS, with a 1-mm mirrored surface,a 50-μm scan of the mirror to the left (X=50 μm), produces a 1.99 mmincrease in optical path length, which is equivalent to 6.6 ps timedelay in free space.

[0035] The figures show the incident optical beam arriving at a90-degree angle to the vertical mirror, In actuality, there are severalincident angles that contain solutions for retroreflection.

[0036] Reflectivity of the reflecting surfaces is below 100% and it istypically 99% and higher, such as 99.99%. The reflective surfaces aretypically made from fused silica, sapphire or diamond, or any othersuitable material, and for MEMs applications silicon can be used.Dielectric coatings on the surface of these materials can be used toincrease their reflectivity to greater than 99.99%.

[0037] The device disclosed herein is scalable, due to its small numberof components, in a simple geometrical arrangement making it useful inmicrooptics, monolithic integration, optical MEMs, macroscopic scaleoptical components, and it can be fabricated on a wide variety ofmaterials.

[0038] The device can be made from a single material polished and/orcoated for increased reflectivity. The device can also be made from anoptically amplifying medium, as in block form, to amplify the light asit passes from the port of entry to the port of exit of the device.

[0039] While presently preferred embodiments have been shown of thenovel retroreflector and method for its operation or use, and severalmodifications discussed, persons skilled in this art will readilyappreciate that various additional changes and modifications can be madewithout departing from the spirit of the invention as defined anddiffereniated by the following claims.

What is claimed is:
 1. A device comprising at least three reflectingsurfaces arranged at or more angles to each other and at least one portwhereby an input optical beam can enter said device through said atleast one port and reflect from the surfaces before exiting said deviceas an output beam through said at least one port along a path that isparallel to the path of the input optical beam.
 2. The device of claim 1wherein said device has 1-2 ports and wherein said ports are disposed inone of said reflecting surfaces.
 3. The device of claim 2 wherein saidports are openings in one of said reflecting surfaces and saidreflecting surfaces are rectangular.
 4. The device of claim 3 whereinsaid device comprises three reflecting surfaces arranged at one or moreangles to each other forming a structure that is triangular incross-section with one port disposed at a corner of one of saidsurfaces.
 5. The device of claim 3 wherein said device comprises threereflecting surfaces arranged at one or more angles to each other forminga triangular structure in cross-section and two ports disposed in one ofsaid surfaces.
 6. The device of claim 5 wherein said two ports aredisposed along one edge of one of said surfaces.
 7. The device of claim4 wherein at least one of said surfaces is spaced from said device onorder to change path length of an optical beam reflected from saidsurfaces.
 8. The device of claim 6 wherein said input optical beamenters said device through one of said two ports, and said outputoptical beams exits said device through the other of said two ports. 9.A retroreflector comprising three internally reflecting surfacesarranged at an angle to each other forming a triangular structure incross-section enclosing space and 1 to 2 ports for inlet and/or outletoptical beam.
 10. The retroreflector of claim 9 wherein said ports aredisposed in one of said surfaces, one of said two ports is disposed at acorner of one of said reflecting surfaces, and said reflecting surfacesare rectangular.
 11. The retroreflector of claim 9 wherein saidretroreflector contains one port, said reflective surfaces arerectangular, and said port is disposed at a corner of one of saidsurfaces.
 12. The retroreflector of claim 11 wherein at least one ofsaid reflective surfaces is spaced from said retroreflector in order tochange length of an optical beam reflected from said surafaces.
 13. Theretroreflector of claim 12 wherein said reflecting surfaces havereflectivity of at least 99% and are made of fused silica.
 14. Theretroreflector of claim 12 wherein said reflecting surfaces are madefrom a material selected from the group consisting of fused silica,sapphire, diamond and mixtures thereof and mixtures containing at leastone of the materials.
 15. The retroreflector of claim 89made in blockform from a single material, wherein said reflective surfaces comprisesides of said block and said ports are enhanced surfaces which are lessreflective than said reflective surfaces.
 16. The retroreflector ofclaim 9 wherein said reflecting surfaces are coated with a film of areflective material to enhance reflectivity of said surfaces.
 17. Theretroreflector of claim 15 wherein said material is optically amplifyingthat can amplify light entering said retroreflector through one of saidports.
 18. The retroreflector of claim 8, wherein said enclosing spaceis a hermetically sealed enclosing space for containing a sample fluid.19. A method of using a retroreflector made of at least three reflectingsurfaces arranged at an angle to each other and at least one portwhereby an optical beam can enter and/or exit the retroreflector throughat least one of the ports, the method comprising the step of passing aninlet optical beam into the retroreflector through one of the portswhereby the optical beam is multiply reflected from the reflectingsurfaces and exits the retroreflector along a path that is parallel tothe path of the inlet optical beam.
 20. The method of claim 19 whereinthe retroreflector comprises three reflective surfaces arranged at anangle to each other and 1 to 2 ports for allowing optical beam to enterand/or exit the retroreflector, the reflecting surfaces forming a prismthat is triangular in cross-section, the method further includes thestep of adjusting at least one angle between the reflecting surfaces inorder to change path length of the optical beam reflected from thereflecting surfaces.
 21. The method of claim 20 including the step ofdisplacing at least one of the reflecting surfaces in order to changepath length of the optical beam reflected from the reflecting surfases.22. The method of claim 19 including the step of hermetically sealingspace formed by the reflecting surfaces for enclosing a fluid in theretroreflector for spectroscopic testing.
 23. The method of claim 19wherein the retroreflector comprises three reflecting surfaces and twoports, the method further includes the step of allowing the optical beamreflected from the reflecting surfaces to exit through a port other thanthe port of entry, reflectivity of the reflecting surfaces is at least99% and the reflecting surfaces are made from a material selected fromthe group consisting of silica, sapphire, diamond and mixtures thereofand mixtures containing at least one of these materials.
 24. The methodof claim 19 wherein the retroreflector comprises three reflectingsurfaces and one port, the method further including the step of allowingthe optical beam reflected from the reflecting surfaces to exit throughthe same port through which the optical beam entered the retroreflector,reflectivity of the reflecting surfaces is at least 99% and thereflecting surfaces are made from a material selected from the groupconsisting of silica, sapphire, diamond and mixtures thereof andmixtures containing at least one of these materials.
 25. The method ofclaim 19 including the step of continuously moving at least one of thereflecting surfaces toward or away from the retroreflector in order tocontinuously change the optical path of the optical beam entering theretroreflector.